Investigation of Conductivity Relaxation and Nonlinear Optical Activity in Pure and Cu2+ Doped Lithium Sulphate Monohydrate Crystals

Abstract

Single crystals of pure and 0.5 mol% Cu²⁺ substituted lithium sulphate monohydrate (LSMH) were grown by slow evaporation. The novelty of this work lies in establishing a defect-assisted conduction model correlating Cu²⁺ substitution, lithium vacancy formation, impedance relaxation, surface morphology, nonlinear optical efficiency, and laser damage resistance. Structural substitution is inferred from ionic size compatibility and systematic electrical response modification. Complex impedance spectroscopy (323–353 K, 20 Hz–2 MHz) reveals bulk-dominated conduction exhibiting non-Debye relaxation. The data were fitted using an equivalent circuit model consisting of Rb–CPE elements, and fitting parameters are reported. AC conductivity follows Jonscher's power law, confirming thermally activated hopping via vacancy defects. A defect chemistry model based on Cu² substitution at Li⁺ sites is proposed to explain conductivity changes. AFM reveals increased roughness upon doping. Second harmonic generation (SHG) was confirmed using the Kurtz–Perry method, and laser damage threshold (LDT) values (∼3 GW/cm²) indicate high optical stability. The combined electrical–defect–optical correlation establishes Cu²⁺ substituted LSMH as a promising multifunctional photonic material.